This invention relates to sailing on snow and ice, and in particular, a ski that efficiently utilizes wind energy for propulsion on snow or ice.
On a hypothetically frictionless surface, the wind blows sailing craft (and all other objects) straight downwind with a speed equal to the wind speed. No other course or speed is possible. In the real world, sailing either upwind or across the wind requires reacting the force generated by the wind on the sail against a surface medium such as land, water, snow or ice. One measure of sailing efficiency is the Foehn number, defined herein as the speed of the sailing craft relative to the speed of the wind. Sailing efficiency increases as the ratio of the lateral force to the drag force of the sailing craft increases, where the lateral force is defined as the force generated by the reaction of sailing craft on the surface medium perpendicular to the direction of travel and the drag force is defined as the resistance in the direction of travel. Hyperwind sailing at Foehn 3 (that is, sailing at three times faster than the wind speed) is readily achieved on sharpened metal blades that are capable of very high lateral/drag force ratios when sliding on smooth ice.
Sailing craft designed to sail on snow have been less successful because of the severe demands required of a sailing ski. The ski must provide vertical lift to support the combined weight of the sailor(s), ski(s) and rig. The ski must also generate the necessary lateral force to counteract the sail forces. In order to achieve high Foehn numbers, the ski must accomplish both tasks with minimum drag in the direction of travel.
Specialized sailboards, monoskis, snowboards as well as iceboats and other boards fitted with a plurality of skis have been invented for sailing on snow or ice but are deficient to varying degrees in performance, stability or control. Iceboats fitted with a plurality of conventional downhill skis for sailing on snow are so inferior in performance to blades on ice that they seldom exceed Foehn 1 and often cannot turn through the eye of the wind without coming to a stop. Inadequate lateral resistance inherent in some of these designs can be dangerous when it results in sudden and unpredictable spinout at high speed.
Many of these inadequacies may be attributed to the use of conventional downhill skis for purposes for which they were not designed. Downhill skis are elongated planar structures that are designed to provide the vertical lift required to support the weight of a skier in snow while simultaneously minimizing the drag opposing the skier's motion. Unlike sailing craft that require a continuous lateral force or side load to balance sail force, downhill skis often support little or no side load even when turning. The lateral force required for skiing across the fall line is achieved by edging or tilting the skis uphill about their longitudinal axes with the legs or ankles. A turn is initiated by tilting the skis into the direction of the turn. The turning radius is controlled by a complex interaction of ski camber, ski flex and sidecut. Because the tip and the tail of the ski provide more grip on the snow than the center that is cut away, the ski bends into a curve that facilitates turning. Skis with more flex and more sidecut have shorter turning radii. Sidecut is detrimental when travelling in a straight line, however, because skis with greater turning ability produce higher drag.
U.S. Pat. No. 5,451,078 and WO9408670 both describe mechanisms for attaching an articulating sailboard sail to the front of a pair of downhill skis. In both inventions, the skis are worn using conventional ski boots. Lateral force is generated by simultaneously tilting both skis to counteract the sail force. Although each ski operates as designed for skiing across the fall line, the drag is higher than optimum because of the sidecut. This configuration is also uncomfortable because it requires the sailor's feet to be aligned in the direction of travel while the upper body is twisted almost 90 degrees to hold the sail. It may also be unstable and dangerous in the event of a fall because the sail attachments impose additional constraints on the relative motion of the skier's feet.
U.S. Pat. No. 4,601,488 and U.S. 5,931,504 disclose two monoski configurations utilizing a sailboard sail attached to the front of a single ski. In these inventions, lateral force is also generated by tilting the ski opposite to the sail force. The monoski configuration may be more comfortable on a reach because the sailor stands with feet pointed across the direction of travel. With only one ski, performance may be enhanced while stability, control and safety problems are exacerbated. In general, monoskis must stop to change direction.
Numerous other attempts have been made to sail on conventional skis and snowboards with a variety of add-on keels, blades or fins. Patent EP0110798 discloses a snowboard with an central rib like an integral keel used in a similar configuration to a monoski. Patent DE3036503 describes a sailboard with rear mounted fins that is also sailed like a monoski. Although probably more stable than a monoski, these inventions likely produce higher drag.
Other configurations have been disclosed that use pairs of downhill skis in a fixed orientation. Patent FR2610837 describes two skis rigidly attached to a central platform that uses an articulating sail. U.S. Pat. No. D 281769 describes a 3-ski (iceboat) configuration using a pair of fixed parallel skis with one central steering ski.
With two rigid skis, lateral force is often generated by fixing one ski with a positive tilt and the second ski with negative tilt of equal magnitude. So oriented, only one ski of each pair effectively produces lateral force while both skis produce drag. The resulting lateral to drag force ratio is poor. The fixed orientation of the skis also inhibits effective turning of the sailing craft because only one ski can flex as designed for skiing. Turning is further inhibited because a conventional downhill ski has both camber and sidecut that causes the tip and tail to dig into the snow and ice when turning.